The present invention is directed to a process for extrusion blow molding extrusion grade polyethylene terephthalate on a high-output blow molding machine such as, for example, a rotary wheel blow molding machine. More particularly, the present invention is directed to a hollow blow needle that is particularly designed for use in connection with extrusion blow molding extrusion grade polyethylene terephthalate.
Polymer resins, such as polyethylene terephthalate (PET), are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The myriad of advantages of PET include toughness, clarity, good barrier properties, lightweight, design flexibility, chemical resistance and good shelf-life performance. Furthermore, PET is environmentally friendly since it can often be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles.
There are a variety of production methodologies to produce PET containers. For example, injection stretch blow molding is commonly used to make PET bottles. Of the various methodologies, one-piece PET containers having an integrated handle are commonly formed using extrusion blow molding (EBM). The EBM process includes extruding a polymer resin in a softened state through an annular die to form a molten hollow tube (also referred to herein as a “parison”). The molten parison is placed in a hollow blow mold having a cavity corresponding to the desired shape of the container being formed. Air is injected to inflate the parison against the interior walls of the blow mold. Upon contact with the walls, the parison cools rapidly and assumes the shape of the mold.
Polyesters (which includes PET) are typically classified by inherent viscosity (I.V.) as a measure of molecular weight. To form beverage bottles, “bottle grade” PET having an I.V. of about 0.72-0.84 dl/g, is typically used. Bottle grade PET has linear polymer chains and by design has a melt viscosity that is low enough to enable a faster injection stretch blow molding step with the least resistance to flow. Bottle grade PETs generally cannot be used in the production of larger handleware containers using EBM because of low melt strength. Melt strength is quantified by measuring melt viscosity at very low shear rates (approaching zero shear rate). Low melt strength hinders the ability to form a suitable parison. For example, in a vertical rotary extrusion blow molding machine, the parison extrudes upwards. The lack of melt strength and subsequent lack of parison rigidity will not allow the parison to form upwards. In this case the material will spill out over the sides of the head when extruded.
To make PET suitable for EBM, PET manufacturers have developed special grades of PET also referred to as extrusion grade PET or “EPET”. Typically, EPET is high molecular weight PET having an I.V. of 1.0 dl/g or greater as measured by solution viscosity. For PET resins I.V. is used as a measure of molecular weight. The average molecular weight of a resin reflects the average length of polymer chains present therein. In general, melt strength increases with chain length and, thereby, also increases with molecular weight. However, higher I.V. polymers generally require higher processing temperatures which lead to certain processing challenges. One major processing challenge is to increase the production output of an EBM process because the process window for a high I.V. PET in an EBM process is narrow, making it difficult to run a stable extrusion blow molding operation at a high production output.
For example, current EBM processes for EPET containers is to employ a shuttle-type blow extrusion blow molding machine. The steps required for a shuttle machine to blow mold a hollow plastic object can be described by the following sequence of operations. First, as the dropping parisons approach the length of the object to be blown, the mold, in an open position, “shuttles” sideways to a point directly under the flow head of the machine. The molds then close to capture the parison. A knife cuts the parisons directly above the molds. The knife may be either a cold knife (cutting with a sharp edge) or a hot knife (burning through the parison).
The molds shuttle away from the flow head until they are directly under the blow pin stations. If the mold movement is horizontal, the extruder head is made to bob up vertically, so that the continuously extruding parisons do not drag against the mold as it moves sideways. In some shuttle machinery, the molds shuttle down at an angle, eliminating the need for the head and extruders to bob upwards. The blow pins are forced down into the still-open necks of the containers, calibrating the necks of the containers. In most cases, the blow pins punch down onto striker plates, which form the top edge of the neck to a precise flat dimension. Air pressure is applied to blow the containers. In many cases, the blow air is turned on before the blow pins enter the open neck of the parison, to force the plastic outward and ensure a good neck formation.
After the containers have cooled, the molds open, and again shuttle under the flow head of the machine. As the molds close on the molten parisons, masking stations that are attached to the sides of the mold close over the outside of the previously blown containers, which are still held in place by the blow pins. The blow pins retract, leaving the containers held only by the masks. As the molds again shuttle sideways, the masks transfer the formed containers sideways to a punching station. Punches come forward to remove the tails, top moil, and any handle (grip) slugs away from the bottles. The bottles are then conveyed out of the machine. This may be done by transferring the bottles onto conveyor belts, by takeout devices, or by simply dropping the bottles into a chute or onto a takeaway conveyor.
A major limitation of a shuttle machine is that it is not cost effective for extremely high volumes such as that experienced in a rotary wheel extrusion blow molding machine such as, for example, a vertical rotary blow-molding machine. A shuttle machine typically has an output of between 20 and 40 bottles per minute (BPM). A vertical rotary blow-molding machine, in contrast, can have an output of, for example, over 100 bottles per minute depending on the number of cavities and molds.
Vertical rotary blow-molding machines index circumferentially spaced mold halves in steps around a vertical axis. In such machines, the flow head typically does not move and extrudes a continuous parison that is continually captured by each mold. The mold opens up just big enough to allow for head to pass through, then captures the parison. The parison is severed by knives attached to the mold as the mold closes. The parison is severed adjacent the top of the mold halves, the mold halves are moved away from the extrusion station, and a “blow needle” is interjected into the side of the mold (near the top) and pierces the parison. The blown parison cools as the mold halves are rotated around the machine, following which the mold halves open at an ejection station and the finished article, commonly a container, is ejected from between the mold halves. In operation, the rotary wheel can produce at least 110 bottles per minute for a 22 cavity mold machine (22 molds) rotating at a rate of 5 RPM.
EPET, however, needs to be processed at higher parison temperatures relative to regular PET (from about 550-600° F.). This necessitates that the molds be kept at higher temperatures (70-80° F.) during the blow molding process to, in part, achieve good container clarity. The challenge in adapting the EPET material to the faster rotary wheel is to control the rheological properties of the molten EPET at each point along the blowing process.
One particular problem addressed by the present invention involves the blow needle and subsequent injection of a fluid such as, for example, air. Because of the high processing temperature of the EPET and the high mold temperature, the semi-molten EPET has poor flow properties. When a conventional blow needle pierces the wall of, for example, a blow dome, and injects pressurized air into the closed mold, the pressurized air can cause the walls of the blow dome to thin to the point where they form a hole through which pressurized air can escape through the mold before the object is completely formed against the mold cavity thus severely deforming the container.
Accordingly, there is a need in the art for a blow needle that can be employed in a system for extrusion blow molding EPET containers on a high-output vertical rotary wheel blow molding machine that will allow for an increased rate of production without the aforementioned drawbacks.